The present invention relates to a fuel injection system for controlling air-fuel ratio of mixture supplied to an engine of a vehicle, and more particularly to a control system for injecting an interrupt fuel during an ordinary fuel injection at acceleration while an air-conditioner is used.
In a known fuel injection system for a motor vehicle, a basic injection pulse width T.sub.p is calculated in dependence on an air flow Q, and engine speed N (T.sub.p =K.times.Q/N, where K is a constant). An actually injected injection pulse width T.sub.i is determined by correcting the basic injection pulse width T.sub.p in accordance with engine operating conditions such as idling and wide open throttle.
In order to compensate for response delay of an air-flow meter for detecting the air flow Q at the acceleration of the vehicle, Japanese Patent Laid Open No. 60-17247 discloses a control system wherein an auxiliary fuel is injected when a differential of pressure in an intake pipe is larger than a predetermined value, which indicates the acceleration of the engine. An auxiliary fuel injection pulse width T.sub.ACC for acceleration is obtained in dependence on a predetermined correction coefficient K.sub.ACCL for acceleration and correction constant T.sub.CONST (T.sub.ACC =K.sub.ACCLK .times.T.sub.p +T.sub.CONST).
However, correction coefficient K.sub.ACCL and constant T.sub.CONST are set to have a proper air-fuel ratio under operating conditions without loads such as an air-conditioner. Accordingly, the power of the engine decreases when the air-conditioner is used. In order to maintain the same power, the driver of the vehicle depresses an accelerator pedal so that the opening degree of the throttle valve is increased to induct more air to increase the engine speed. The increase of the air flow Q causes an increase of the basic fuel injection pulse width T.sub.p. However, since the coefficient K.sub.ACCL and the constant T.sub.CONST are set irrespective of the operation of the air-conditioner, auxiliary fuel injection pulse width T.sub.ACC also increases with increase of the engine speed. Thus, air-fuel mixture becomes excessively rich to reduce the combustion efficiency, thereby causing hesitation or stumble at the start and acceleration of the vehicle and hence decreasing the driveability.